Yes, that's the "going round and round" bit I alluded to, an old argument as hoary as Balrog wings. It starts with a given datum: that once Frodo received the ring on his 33rd birthday he stopped physically aging.

That's not what I meant at all (okay, just a little). There isn't a seventeen year gap in the films between Bilbo's party (on September 22, 3000 as per FotR-EE) and Frodo's departure from the Shire. It is probably no more than a gap of one year, with Frodo reaching the Ford of Bruinen on 20 October, 3001. Frodo was physically much younger when he left the Shire in the films than he was in Tolkien's legendarium. The timeline of the movies is all messed up.

Still, strictly in terms of age, I have to agree that Elijah Wood would have been better cast as Pippin who, in the book, was the youngest member of the company. I can't fully agree with Christopher Tolkien, though, on the subject of hobbit maturity rates. I can't think of any human society, no matter how highly structured, where a man's coming of age wasn't recognized until he had reached his thirties. I am convinced that J.R.R. Tolkien's intent was that hobbits reached physical maturity around the time of their thirty-third year. On the other hand, there is no reason to think that this had to be extended as well to the Dúnedain. "Change is inevitable. Growth is optional." - DRWolf (after John C. Maxwell)